Conjugated cyclic molecule

The meaning of the word aromaticity has evolved as understanding of the special properties of benzene and other aromatic molecules has deepened. Originally, aromaticity was associated with a special chemical reactivity. The aromatic hydrocarbons were considered to be those unsaturated systems that underwent substitution reactions in preference to addition. Later, the idea of special stability became more important. Benzene can be shown to be much lower in enthalpy than predicted by summation of the normal bond energies for the C=C, C—C, and C—H bonds in the Kekule representation of benzene. Aromaticity is now generally associated with this property of special stability of certain completely conjugated cyclic molecules. A major contribution to the stability of aromatic systems results from the delocalization of electrons in these molecules. 

Cyclazines are conjugated cyclic molecules in which planarity is secured by three covalent bonds to a central nitrogen atom. They differ in the size of the individual ring units. In the nomenclature of cyclazines, the name is preceded by the number of jt-centers between the points of attachment to the nitrogen which are arranged by increasing number and put in brackets, e.g. [3.3.3]cyclazine (1) and [2.3.4]cyclazine (2). 

Magnetic criteria of aromaticity are based on the model of interatomic teratomic ring currents induced in conjugated cyclic molecules by external magnetic fields. 

Dewar et al.83 showed that the energy of linear polyenes is an additive quantity of bond energy increments. This enabled Dewar to quantify the thermochemical stability or instability of a cyclic molecule relative to a reference molecule. The reference molecule R for a conjugated cyclic molecule... 

The same difference can be shown to distinguish between cyclic systems with 4m and 4m + 2 carbon atoms respectively. The n = 0 energy level has degeneracies of m and 2m +1, for these respective systems. It is apparent that the zero level of 4m cyclics cannot accommodate more than 2m itinerant electrons, which means that the 2m odd couples cannot be delocalized to yield an aromatic system. The best known example of a non-aromatic conjugated cyclic molecule is cyclo-octatetraene, shown in Figure 6.5. 

It may be noted that ring current effects are only discernible in planar conjugated cyclic molecules. In nonplanar molecules, e.g., cyclooctatetraene (Figure 1.21), which is tub-shaped, the delocalization of 7i-electrons is severely restricted, and the protons of cyclooctatetraene therefore resonate in the normal olefinic region (5 5.80, cf. 5.85 in cyclohexadiene). 

Both thermochemical and MO approaches agree that benzene is an especially stable molecule and are reasonably consistent with one another in the stabilization energy which is assigned. It is very significant that MO calculations also show a destabilization of certain conjugated cyclic polyenes, cyclobutadiene in particular. The instability of cyclobutadiene has precluded any thermochemical evaluation of the extent of destabilization. Compounds that are destabilized relative to conjugated noncydic polyene models are called antiaro-maticf ... 

Orbitals interact in cyclic manners in cyclic molecules and at cyclic transition structures of chemical reactions. The orbital phase theory is readily seen to contain the Hueckel 4n h- 2 ti electron rule for aromaticity and the Woodward-Hof nann mle for the pericyclic reactions. Both rules have been well documented. Here we review the advances in the cyclic conjugation, which cannot be made either by the Hueckel rule or by the Woodward-Hoffmann rule but only by the orbital phase theory. 

Scheme 22. Definition of a Reference Structure, R, Which Is a. T-Isoconjugate to a Cyclic Molecule (CM) but with a Degree of Conjugation of an Open-Chain Polyene...

CM) is shown in Scheme 22, and it is seen to involve cyclic conjugation, but all the interactions between the "T-bond units have a polyenic character. The relative energy of the two structures, as in eq 7, is defined as the Dewar resonance energy (DRE) of the cyclic molecule. 

A series of SAMs formed on Au from mono- and dithiol conjugated aromatic molecules was characterized by cyclic voltammetry, grazing incidence Fourier transform infrared spectroscopy, contact angle measurement, and ellipsometry.43 The analyses indicated that the molecular orientation of conjugated phenylene- and thophene-based dithiols became less tilted with respect to the surface normal as the chain length of the organic molecules increased. 

For l,3,2(A2)-diazasilacyclopentene 83, the following Raman lines at 650, 991, 1178, 1566, 1573 cm-1 were reported in the solid state. The intensive doublet at 1566 and 1573 cm-1 was assigned to the (C=C) stretch vibration. The observed bathochromic shift of this band in the spectra of compound 83 compared to heterocyclic compounds having a tetravalent silicon, for example, 93, and the significant intensity enhancement of this band indicates cyclic conjugation in molecule 83 <2000JST329, 2004JA4114>. 

Cyclic, fully conjugated, planar molecules with 4n + 2 pi electrons (n = any integer including zero) are aromatic. 

The criteria for an antiaromatic compound can be generalized in a similar manner. The requisite number of electrons to have the HOMOs half filled is a multiple of four (an even number of pairs). Therefore, cyclic, fully conjugated, planar molecules with 4n pi electrons are antiaromatic. Some of the possible numbers of electrons are as follows ... 

Anti-aromaticity was predicted by the Hiickel approach for conjugated cyclic planar structures with 4n 7i electrons due to the presence of two electrons in antibonding orbitals, such as in the cydopropenyl anion, cydobutadiene, and the cydopentadienyl cation (n = 1), and in the cydoheptatrienyl anion and cydooctatetraene (n = 2). It has been argued that a simple definition of an anti-aromatic molecule is one for which the 1H NMR shifts reveal a paramagnetic ring current, but the subject is controversial. The power of the Hiickel theory indeed resides not only in the aromatic stabilization of cydic 4n + 2 electron systems, but also in the destabilization of those with An electrons [22, 27, 42]. 

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